1. Field of the Invention
The present invention relates to a chip resistor comprising an insulating substrate in the form of a chip, at least one resistor film formed on the substrate, terminals provided at opposite ends of the resistor film, and a cover coat covering the resistor film.
2. Description of the Related Art
Conventionally, in a chip resistor of the above-described type, the cover coat covering the resistor film projects largely from a central portion of the upper surface of the insulating substrate, thereby providing stepped portions in the chip resistor. The stepped portions cause such a problem that a collet of a vacuum suction type cannot duly suck the chip resistor or that the collet may break the cover coat when the chip resistor is to be mounted on a printed circuit board.
There is another problem as follows. The terminals provided at opposite ends of the resistor film include upper electrodes formed on the upper surface of the insulating substrate for connection to the resistor film. Generally, the upper electrodes are made of conductive paste containing silver as the main ingredient. (Hereinafter, the paste is referred to as “silver-based conductive paste,” or more simply “silver paste”). Therefore, although the upper electrodes are covered with metal plating layers, corrosion such as migration due to e.g. sulfur in the air may occur at the upper electrodes. Specifically, silver in the upper electrodes reacts with sulfur gas such as hydrogen sulfide in the air to form silver sulfide. Such corrosion may lead to a break in the upper electrodes.
JP-A-2002-184602 discloses a chip resistor capable of solving the above-described problems. Specifically, in the disclosed chip resistor, an auxiliary upper electrode made of nickel-based conductive paste, or nickel paste, which does not contain silver is formed on each of the upper electrodes to partially overlap the cover coat. With this arrangement, the difference in height at the stepped portions is eliminated or reduced, and the corrosion of the upper electrodes are prevented.
A chip resistor of the above type, in addition to the upper electrodes, includes a side electrode made of silver paste, the side electrode provided on a side surface of the insulating substrate for connection to the upper electrode. For facilitating the soldering of the chip resistor, the upper electrode and the side electrode are covered with a metal coating consisting of an underlying nickel-plated layer and an upper layer formed by tin-plating or solder-plating on the nickel-plated layer.
The provision of the nickel-plated layer as the base is important for preventing the upper electrode and the side electrode from being eroded due to soldering. In light of this, conventionally, it is necessary to check, after the nickel-plated layer forming step, whether or not the desired nickel layer has been formed. Since nickel is a ferromagnetic metal, an accurate check for the formation of the nickel-plated layer can be made at low cost with a relatively simple checking system utilizing a magnet.
However, when the auxiliary upper electrode made of nickel paste is formed on the upper electrode as disclosed in JP-A-2002-184602, the formation of the nickel-plated layer cannot be checked with the above-described checking system utilizing a magnet. Instead, the checking need be performed with the use of a more complicated and expensive system.
When the auxiliary upper electrode is made of a silver or copper paste, it may be corroded by the airborne sulfur compounds, so that the corrosion of the upper electrode cannot be prevented completely.